The following art defines the present state of this field:
Heron, U.S. Pat. No. 1,953,229 describes an invention that relates to spark plug electrodes. This invention provides a spark plug electrode which will resist the corrosion and erosion produced by modern fuels embodying tetraethyl lead and which has a long period of usefulness without requiring replacement or readjustment.
Weber, U.S. Pat. No. 3,753,795 describes a spark plug electrode that is made of a dispersion-strengthened nickel alloy containing small amounts of aluminum and, optionally, chromium.
Heywood et al., U.S. Pat. No. 4,081,710 describes an igniter, particularly for gas turbine engines, and comprising two or more electrodes separated by a body of insulating or semi-conducting material and having exposed working surfaces between which sparks may pass, at least part of the working surface or surfaces of at least one of the electrodes comprising a host material in which Co or Ni predominates alloyed or compounded with one or more additional metals selected from the group consisting of Ru, Rh, Pd, Ir, Pt, Ag and Au. Preferably, the additional metal is platinum which is present in an amount of 1 to 20 wt. % of the total metal content.
Nishio et al., U.S. Pat. No. 4,400,643 describes a spark plug that includes an insulator body with a center bore and a bottom end defining a discharge end of the insulator body and a discharge center electrode formed in a region of the discharge end of the insulator body; a spark plug including thermal conductivity-controlling material comprising spherical metal powder as an essential element thereof in the center bore providing function to control thermal conductivity of the spark plug. The conductivity controlling material further comprises refractory powder and glass powder. The controlling material is also composed of spherical metal powder coated with a ceramic layer of a mixture thereof with the spherical metal powder. The spark plug with the controlling material permits and increasing conductance according to temperature rise to provide a thermally wide-ranged spark plug.
Sato et al., U.S. Pat. No. 4,853,582 describes a spark plug for use in internal combustion engines having a pair of electrodes between which electric spark discharge is effected. The spark plug has a spark discharge portion bonded by, for example, resistance welding to at least one of the electrodes and made of a base metal containing at least 90 wt % of chromium (Cr). A stress-relieving portion having a thermal expansion coefficient intermediate in value between those of the electrode and the spark discharge portion may be formed between the electrode and the spark discharge portion.
My issued patent U.S. Pat. No. 5,569,971 to Clifford et al. defines a spark electrode and method of assembly and is hereby incorporated into this application by reference. In this patent is described an assembly similar to that of the instant disclosure. However, the present invention provides several important advantages over the '971 reference as will be described below.
Almost all spark electrodes, from those used in combustion engine spark plugs to gas heater igniters, have a central metallic conductor housed in a ceramic or glass insulator. A fundamental difficulty with such electrodes has been that the thermal expansion rate of the interior electrode is greater than that of the surrounding ceramic insulator. As such, as the electrode becomes hotter during use and expands, it applies pressure to the inside wall of the insulator. Such ceramic and glass insulators are by their nature extremely brittle, and thus are prone to cracking under thermal expansion pressure from the electrode. A further problem with such prior art devices is that the electrode must be firmly mounted within the housing in such a way that it will not become loose over time. The methods heretofore used for assembling the electrode to the housing are at once expensive and often require more components than just the electrode and ceramic housing.
Prior art devices are available for reducing the chance of structural failure during operation and which provide unique methods for holding the electrode in place within the ceramic housing. Such devices typically teach that the electrode is to be fixed to the ceramic housing by braising processes, or by melting the ceramic or glass housing around the electrode and letting the molten glass set the electrode into place. Other prior art devices teach a spark electrode assembly having multiple parts that cooperate to hold the electrode in place. All of these prior art solutions are relatively expensive, and many require more than two parts. As such, the prior art devices tend to be relatively complex, and thus more expensive and more prone to failure.
Clearly, then, there is a need for a spark electrode device that can be readily assembled and that comprises only the two basic parts of the electrode and the insulating, ceramic housing. Such a needed device would accommodate the thermal expansion of the electrode. Such a needed device would further allow for the thermal contraction of the electrode while still firmly holding the electrode in place within the housing. The present invention fulfills these needs and provides further related advantages such as being very inexpensive to manufacture in scale.